I think it was Niles that made a unit that would do the trick for you but I don't think it is made any longer. I have one and will have to look for it. It isn't on their website now. Send me an e-mail and let me know if you want me to look for it. I don't think the preamp will be hurt by pulling a little extra current to power on the amp input relay. Chances are it takes way less power than what is specified but may require the additional current only at startup.
Brad
Brad